ok, after having heard lots of opinions on the subject i thought i’d spend a
minute or two transcribing an authoritative definition of amplifier class
(specifically tube amplifiers) for everyone. this comes from “reference data
for radio engineers”. definitions in the 1968 rca receiving tube manual are
nearly identical.

class ab: grid bias and alternating grid voltages such that plate current
flows appreciably more than half but less than entire electrical cycle.
(180 < theta-p < 360 degrees)

class b: grid bias close to cutoff such that plate current flows only during
approximately half of the electrical cycle. (theta-p approx 180 deg.)

class c: grid bias appreciably greater than cutoff so that plate current flows
for appreciably less than half of the electrical cycle. (theta-p < 180 deg.)

a further classification between circuits in which positive grid current is
conducted during some portion of the cycle and those in which it is not, is
denoted by subscripts 2 and 1, respectively. thus, a class ab2 amplifier
operates with a positive swing of the alternating grid voltage such that
positive electronic current is conducted and accordingly in-phase power is
required to drive the tube.

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note that class of operation is silent on the subject of circuit topology.
that is, you may have a push-pull topology that operates in class a, b, ab1
or ab2.

the rca receiving tube manual of 1968 demonstrates both parallel and push-pull
topologies for a class a power amplifier and gives a reasonable discussion of
the tradeoffs involved. they also demonstrate class ab, ab1, ab2 and b
in push-pull topology. they do not discuss class c audio amplifiers.

now… there’s no universally-accepted definition of “single-ended”.
customarily, it has been used to distinguish between those outputs -or-
inputs that are ground referenced and those that are not (ie, differential).
single-ended carries no connotation of operating class whatsoever, nor of
topology of power amplification.